A growth process of chemical vapor deposited Titanium-Carbide coating layer on sintered WC-Co substrate is studied at varing deposition conditions and Co content of the substrate.
Nucleation and initial growth is facilitated by increasing Co content of the substrate for both the substrate reaction and the hydrocarbon reaction. The experimental result indicates that the nucleation is controlled by the diffusion of carbon through Co rich phase. It is observed that initial nucleation site is the interface between WC grains and Co rich phase. The rate of surface coverage is increased by $\frac{3}{2}$ power with time for the overall reaction.
During the further growth stage of substrate reaction, the growth rate is increased linearly with square root of time. Increasing Co content of the substrate has pronounced effects on the growth rate and the activation energy. The experimental result reveals that the growth mechanism of substrate reaction is controlled by the diffusion of carbon.
For the overall reaction, the growth rate is increased linearly with time. However the growth rate and the activation energy are not varied with Co content. The experimental result provides an evidence that the growth mechanism of the hydro-carbon reaction is controlled by the chemical reaction. An emperical equation is developed for the dependence of growth rate on deposition temperature and total pressure.
Scanning electron microscopy revealed that the grain shade was equiaxed for all deposition conditions. Also it revealed that grain size was finer when the deposition temperature and total pressure were decreased, and when the carbon concentration in the reactant gas was increased. It is believed that the grain refinement is due to the enhanced nucleation.
It is found that the growth rate of η layer is increased with that of TiC layer. The carbon concentration in the reactant gas has pronounced effects on the growth rate of η layer. A schematic model is suggested for the formation and growth mechanism of η layer.
The result of this experiment suggests an interrelationship between preferred orientation, microhardness and coating structure. The specimen deposited below 1050℃ has ＜100＞ ＜111＞ preferred orientation and finer equiaxed grains, resulting in higher microhardness and much higher value with decreasing total pressure. But the specimen deposited above 1050℃ has ＜110＞ preferred orientation and coarser elongated grains resulting in lower microhardness values.
